Therapeutic proteins have become very successful in the treatment of various chronic and life-threatening diseases. However, besides their benefits, therapeutic proteins seem to have a common problem - the response of a patient’s immune system against the protein. This means that the immune system of the patient actively removes the drug from the body, thereby potentially decreasing or reversing the effect of the therapy. By now there is strong consensus that damaged and aggregated proteins are important risk factors. Protein aggregates are, due to their heterogeneity and often low quantity, challanging to characterize. Further, there is a large academic interest in understanding the mechanisms of aggregation and the role of non-proteinaceous particles in the process of protein aggregation and unwanted immunogenicity in order to design more effective and safe protein-based medicines. This PhD thesis supported that research effort by developing and improving analytical methodologies to detect the size, quantity and other properties of protein aggregates and particles, especially in the relevant nano- and micrometer size range. These techniques were then applied to study a so far unknown nanoparticulate impurity in pharmaceutical-grade sugars. Further, the results shown in this thesis revealed that these nanoparticulate impurities pose a threat to protein stability.